Use of a diffractive optical element in a microscope optical system provides advantages in chromatic aberration correction, and the like, over the prior art, and makes if possible to design a low-cost microscope objective lens with high optical performance. However, when a diffractive optical element is used in a microscope optical system, not only the order of diffracted light contributing to the image formation but also the other orders of diffracted light (hereinafter referred to as “unnecessary-order diffracted light”) pass through the microscope objective lens, and hence flare is caused. It is known that, in order to obscure the flare caused by the unnecessary-order diffracted light exited from the diffractive optical element, it is only necessary to increase the size of the spot, which is formed on the image surface by the unnecessary-order diffracted light (see, for example, Patent Literature 1).